The present invention is directed to the use of a substantially non-porous membrane for solvent extraction employing two substantially immiscible liquids. The novel process requires no external pressure gradient to effect extraction and has many advantages over the conventional two-phase solvent extraction.
Solvent extraction has been a useful operation in separation processes. In the operation, a liquid solvent C is used to extract a solute (or solutes) A from a second liquid B in which A is dissolved. Solvent liquids B and C are immiscible or substantially immiscible.
For a conventional operation, solvent liquids B and C are mixed directly, then separated into two phases. During the direct mixing, however, certain inherent difficulties are present. To illustrate, a foam frequently develops which has the disadvantages, for example, of reducing the mass-transfer rate and prevents a complete phase separation. To increase mass-transfer area in conventional solvent extraction, small drops have to be formed during the mixing. In such cases, the power consumption in processes involving a mixture of the liquid phases is high due to such operations such as mixer-settler, mechanically stirred column, etc. When the drops which occur during the mixing are formed, a back mixing may occur, leading to a reduction of mass-transfer rate. Also, the smaller the size of the drops, the more readily a foam is generated.
In a given stream, if the solute A is to be extracted from a mixture of solutes, the liquid solvent C, for a conventional operation, must be chosen so as to extract A selectively from other solutes in the solvent liquid B. However, sometimes the selectivity becomes difficult to establish. In addition to the limitations of the conventional solvent extraction mentioned above, solvent loss is high owing to entrainment and hold-up tanks are generally required for phase separations. The present invention, using a solvent swollen membrane system, avoids these and various other drawbacks inherent in conventional solvent extraction techniques in which the respective liquid solvents are in direct contact wih each other.